One of the procedures currently in use to perform aerial refueling of aircraft involves the use of a cylindrical beam (boom) with a telescopic tube through which fuel is passed from the tanker aircraft to the receiver aircraft. The boom is articulated with two degrees of rotational freedom at the attachment point to the tanker. A pair of independently moving fins (ruddevators) attached to the boom at a distance to the articulation, allow the boom operators to fly the boom, controlling its position in space, to guide the telescopic tube into the receptacle of the receiver.
The boom cross section must be symmetric and have a low drag, so that for a given control force in the ruddevators a large operational flyable envelope can be achieved. This need for a low drag boom section is even more important when the boom is to be capable to refuel large aircraft, as this operation is performed in specific conditions that increase the drag.
The low thickness-low drag airfoils typically used in aeronautics generate a large amount of lateral force (lift) when they are subjected to local sideslip (incident current out of the section plane of symmetry). For the typical applications of these airfoils, this is beneficial, as the requirement is to have the largest lift to drag ratio possible. Nevertheless, for the particular application of the boom at sideslip, this lateral force is detrimental, as it opposes the control force introduced by the ruddervators, thus reducing the operational envelope of the system. This problem is aggravated by the boom flexibility. The lateral force generated by the ruddevators to control de boom twist the boom in the sense to increase the local sideslip, with the corresponding increment of the opposing force generated by the boom and associated reduction of the flyable envelope.
A possible way to solve this problem is to use circular or quasi-circular boom cross-sections, which generate much less lift than an airfoil, but they also produce a much larger drag than an airfoil, with the corresponding impact on the operational envelope.
Another way to address this issue is by the use of thick airfoils (70% thickness ratio for instance) that when at sideslip (due to a turn as rigid solid of the boom around the articulation or due to its elastic deformation) generate negative lift which adds to the ruddevators control force and does not oppose it. This type of airfoils has several disadvantages:                non-linearities in the lift-incidence relationship, which complicate the control laws design;        larger drag with the corresponding impact on the operational envelope;        high sensitivity to variations in section geometry and flight conditions (Mach number, Reynolds number, elevation angle, turbulence) which complicate control and may even induce the loss of the inverse lift effect;        they are prone to aeroelastic instability problems (divergence, flutter, galloping, etc.).        
The present invention comes to solve these drawbacks.